Close focus correctio for Dall-Kirkham

[jabberwock48]

Hi

We are trying to set up a 14in  Orion Optics UK ODK  at a remote site in New Mexico. We have so far been unable to get the scope to focus as it should. It now looks as though the primary to secondary mirror spacing is not correct to give focus 210 mm from the rear cell as it should be. We know how to set  the spacing using the supplied Ronchi grating using a star. However the telescope is not polar aligned so that will be a bit tricky.

Does anybody know a correction formula for how the focal distance will change from 210 mm for a star at infinity to an artificial star at 15 feet or so. This will allow to set the spacing approximately, Then polar align the scope and the repeat process on a star for a more accurate result.

Thanks for any advice.

John